Leave E on the floor on the ADC’s last-hit step—Lux gets mean when the slow field makes them show their dodge or stay for farm, and only then do you choose the detonation or the follow-up Q
Lux’s real catch is often the two-body bind: fire Q through the front body that the carry is standing behind, because rooting the support, diver, or even a minion in line is frequently how you reach the target you actually want
Recent data fits the feel that Lux support wants to keep repeating those lane and corridor catches before the game settles; she cashes early leads well and looks less comfortable than similar mage supports in long even-or-behind games
Auto for passive when the mark is part of a won sequence, not a tax you pay out of habit. The clean Lux support proc is usually after E has already slowed their path, after Q pinned them, or when your ADC can step with you so the answer trade is short and one-sided. If the enemy support still has hook or all-in, the wave blocks your retreat, or stepping up puts you inside their real range, leave the mark alone and keep your spacing. Missing one passive proc is cheap; donating your health bar or flash is not.
Lux's damaging spells charge the target with energy for a few seconds. Lux's next attack ignites the energy, dealing bonus magic damage (depending on Lux's level) to the target.
Lux releases a sphere of light that binds and deals damage to up to two enemy units.
Lux throws her wand and bends the light around any friendly target it touches, protecting them from enemy damage.
Fires an anomaly of twisted light to an area, which slows nearby enemies. Lux can detonate it to damage enemies in the area of effect.
After gathering energy, Lux fires a beam of light that deals damage to all targets in the area. In addition, triggers Lux's passive ability and refreshes the Illumination debuff duration.